A prior simple demodulation system for a frequency demodulation signal is a discriminator system using an L and a C circuit, or a delay line. In this case, the signal-to-noise ratio S/N of the demodulated signal for the C/N of the FM modulated input signal (C/N is the ratio of the carrier signal power to the noise power) is expressed: EQU S/N=C/N FI
where FI is a constant. Thus, the value S/N of the demodulated signal is proportional to the C/N of the input signal.
On the other hand, the value C/N depends upon the bandwidth of a bandpass filter, which is provided at the input side of a demodulator for restricting the bandwidth of both the noise power and the signal power. Generally, that equation is satisfied when the value C/N is higher than 10 dB, and when the value C/N is less than that value, the value S/N is rapidly deteriorated. That value of C/N (=10 dB) is called a threshold level.
An FM system is utilized for transmitting a television (picture) signal, for instance through a satellite relay system. In this case, the receive level at a receive side is near the threshold level because of the restriction of the transmission power of a satellite, the stability of the transmission path, and/or the cost of the reception equipment at an earth station. Accordingly, an input signal level is decreased by the change of the condition of the transmission path, and that level sometimes becomes lower than the threshold level. Then, the demodulated picture signal on a television screen is deteriorated considerably by an impulse noise which is peculiar to an FM transmission system. And sometimes no demodulated picture signal is obtained.
Accordingly, it is important to improve the characteristics for that impulse noise for improving a television picture and/or reducing the cost of reception equipment, in particular, it is very important to improve the noise characteristics when a direct broadcasting satellite communication system is utilized.
Many systems have been proposed for improving the threshold level. One of them is a dynamic tracking system (see PROC IEE vol.115, No.11, November 1968, pages 1597-1606) as shown in FIG. 1. The reference numeral 1 is a narrow band variable bandpass filter the center frequency of which is controllable, 2 is a limiter, 3 is a frequency discriminator, 4 is a lowpass filter, 5 is a signal input terminal, 6 is an output terminal of the demodulated signal. The FM signal at the input terminal 5 passes the variable bandpass filter 1, the limiter 2, and is demodulated by the discriminator 3. The demodulated signal controls the center frequency of the variable bandpass filter 1 through the lowpass filter 4 which removes the high frequency component noise. The center frequency of the variable bandpass filter 1 follows completely to the instantaneous frequency change of the input signal at the input terminal 5. That is to say, the amplitude component and the phase component of the output of the discriminator is fedback completely to control the center frequency of the variable bandpass filter.
Next, the demodulation of an FM signal of a color television signal is described. A color picture signal has an intensity signal and a color sub-carrier signal, and the total bandwidth of the color picture signal is even 4.2 MHz. Further, the frequency spectrum of a color picture signal depends considerably upon the picture, in particular, the amplitude of the color sub-carrier component depends upon the saturation factor of a color of a picture.
Due to the nature of a color picture signal, a conventional FM signal demodulation system of FIG. 1 has some disadvantages. First, it is almost impossible to feedback the amplitude and the phase of the output of the discriminator through the feedback circuit having an amplifier, a lowpass filter to the variable bandpass filter, stably and following completely to the instantaneous frequency change of the input FM signal, since a television signal has a very wide bandwidth.
If the feedback signal is not complete, an FM signal with a large deviation can not pass the variable bandpass filter, and then, the demodulated waveform is distorted. When the value C/N and/or S/N is low, that waveform distortion is not critical. The higher the value C/N is, the more critical the waveform distortion is. Therefore, when the value C/N is high, a simple discriminator having a fixed bandwidth provides better picture quality than the circuit of FIG. 1 does.
Further, when the frequency deviation is large, and the high modulation frequency component is not completely fedback, that component having the large FM deviation power is removed by the variable bandpass filter, thus, the value C/N is deteriorated, and the value C/N becomes less than the threshold level. Thus, the presence of a variable bandpass filter provides the undesirable effect.
Even when the feedback circuit functions correctly, the input signal does not effectively pass the variable bandpass filter due to the feedback of the wideband noise component. As a result, this noise deteriorates the demodulated picture signal. In particular, when the signal level is rather low, the demodulated picture signal is much affected by the wideband. For instance, when the saturation of the color signal is low, the wideband noise is demodulated, and that demodulated noise deteriorates the picture on the screen by generating the visible color noise.